Recording densities in hard disk drives have been steadily increasing. Indeed, recording densities of 100 gigabits per square inch (Gbit/inch2) have been reported. A requirement for achieving these high densities is to reduce the distance between the magnetic head and the magnetic recording layer of the magnetic disk as much as possible. Currently, this distance is generally 20 nm.
To reduce this distance as much as possible, the surface roughness of the magnetic disk should be reduced as much as possible. Therefore, there has been a transition from the contact start/stop (CSS) systems to load/unload (L/UL) systems. In CSS systems, the magnetic head is in contact with the magnetic disk when the disk is not spinning and the magnetic head flies up due to air currents when the magnetic disk begins spinning. In L/UL systems, the magnetic head is retracted away from the magnetic disk (unloaded) when the disk is stopped and is loaded on to the magnetic disk when the magnetic disk begins spinning. Further, in L/UL systems, anti-sliding characteristics can be relaxed somewhat. The hard disk drive, however, must be able to withstand impacts from load-on operations as well as sudden irregularities in head orientation that can occur even in normal operations.
Traditionally, perfluoropolyether (PFPE) based lubricants have applied been on the top surface of the magnetic disk to reduce friction. However, PFPE based lubricants, such as Zdol and Ztetraol suffer from catalytic decomposition in the presence of Lewis acids, like Al2O3. It is believed that hydrogen fluoride (HF) is generated due to thermal decomposition from friction heat or decomposition, and that this HF causes a chain reaction that leads to further decomposition of the lubricating agent.
Additionally, long chain PFPE lubricants such as ZDol and ZTetraol have a further drawback. Because ZDol and ZTetraol only have functional groups (hydroxyl groups) on the two ends of perfluoropolyether (PFPE) chain, the chain tends to bulk up on the surface of the disk. The bulked up chain results in a lubricant with a high profile.
Furthermore, mixtures of high profile PFPE lubricants with PFPE lubricants having a cyclophosphazene ring group have been reported. However, a mixture of two or more compounds imposes a difficulty for process control, e.g. different lube uptaken between these two compounds, control of the ratio of these two compounds at different locations on the disk, etc. Also phase separation of the compounds has been observed in the case of ZDol/X1P, and ZTetraol/X1P mixtures.